Lasers are used for many applications. For example, lasers, such as KrF and ArF excimer lasers, are used in stepper and scanner equipment for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the steppers and scanners are designed for a particular wavelength of the laser. The laser wavelength may drift over time and, thus, a feedback network is typically employed to detect the wavelength of the laser and correct the wavelength as necessary.
In one type of feedback network used to detect and adjust the wavelength of a laser, an etalon receives a portion of the emitted light from the laser. The etalon creates an interference pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light. The concentric bands surround a center bright portion. The diameter of a light band produced by an etalon is used to determine the wavelength of the laser to a fine degree, such as to within 0.01-0.03 pm. The width of a light band is used to determine the spectral width of the laser output. The interference pattern is usually referred to as a fringe pattern. A grating spectrometer is also used in prior art devices to measure wavelength to a relatively course degree. The fringe pattern and the grating signal may be optically detected by a sensitive photodetector array. A detailed description of a prior art wavemeter is disclosed in U.S. Pat. No. 5,978,394 which is incorporated herein by reference.
Various methods are well known for wavelength tuning of lasers. Typically the tuning takes place in a quickly replaceable modular device referred to as a line narrowing module or line narrowing package (LNP). A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the discharge chamber through which a portion of the laser beam passes into the LNP. There, the portion of the beam is expanded in a beam expander and directed to a grating which reflects a narrow selected portion of the laser""s natural broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a mirror adjustment with a pivoting mirror in the beam path. The adjustment of the grating position or the mirror position may be made by a mechanism which we will refer to as a laser wavelength adjustment mechanism.
In the prior art, the typical feedback network is configured to maintain the nominal wavelength within a desired range of wavelengths. Typical specifications may establish this range at values such as xc2x10.05 pm of a target wavelength such as, for example, 248,327.1 pm, for a KrF laser as applied to the average of the wavelengths of a series of pulses referred to as xe2x80x9cpulse windowxe2x80x9d. A typical pulse window would be 30 pulses. Another typical specification is the standard deviation of the measured wavelength values for a series of pulses (such as 30 pulses). This value is referred to as wavelength sigma, "sgr", and is calculated using a standard formula for standard deviations. Also, sometime specifications are in terms of 3"sgr" which is merely three times the measured standard deviation. A typical 3"sgr" specification may be 0.15 pm.
The limitations of acceptable optical lens materials to fused silica and calcium fluoride for use with deep ultraviolet light at 248 nm and 193 nm wavelengths have meant that projection lenses for KrF and ArF lithography, to a large degree, cannot be corrected for wavelength variations. Chromatic aberrations emerge since the index of refraction of any optical material changes with wavelength, and hence, the imaging behavior of a lens also varies with wavelength.
The detrimental effects of chromatic aberrations for an uncorrected lens can be mitigated by using a light source with a very narrow range of wavelengths. Spectral line-narrowed excimer lasers have served this purpose for deep-UV lithography. In the past, laser specifications have required the FWHM bandwidth to be smaller than a specified value such as 0.5 pm but with no lower limit on bandwidth. Specifications are also directed at the 95 percent integral bandwidth. A typical 95% I specification would be less than 1.2 ppm. However, recently integrated circuit manufacturers have noticed that the quality of their integrated circuits can be adversely affected by bandwidths, such as about 0.35 pm FWHM, which are substantially narrower than the bandwidths for which their optical systems were designed.
A lithography technique, called FLEX (short for, xe2x80x9cfocus latitude enhancement exposurexe2x80x9d) has been shown (through simulation and experiment) to improve the depth of focus by utilizing multiple exposure passes of the same field with different focus settings. This technique is also commonly referred to as focus drilling, since the physical thickness of the photoresist film is exposed in multiple passes at incremental focus settings. The image in photoresist is formed by the composite of the multiple exposure passes.
Several difficulties result from this FLEX process with both step and scan as well as step and repeat exposure implementations. Multiple pass exposure results in additional overlay (image placement) errors and image blurring. This has further implications on process latitude, focus repeatability as well as wafer throughput since multiple exposures require multiple imaging passes.
What is needed is a better technique for providing improved quality integrated circuit lithographic exposures.
The present invention provides an integrated circuit lithography technique called spectral engineering by Applicants, for bandwidth control of an electric discharge laser. In a preferred process, a computer model is used to model lithographic parameters to determine a desired laser spectrum needed to produce a desired lithographic result. A fast responding tuning mechanism is then used to adjust center wavelength of laser pulses in a burst of pulses to achieve an integrated spectrum for the burst of pulses approximating the desired laser spectrum. The laser beam bandwidth is controlled to produce an effective beam spectrum having at least two spectral peaks in order to produce improved pattern resolution in photo resist film. Line narrowing equipment is provided having at least one piezoelectric drive and a fast bandwidth detection control system having a time response of less than about 2.0 millisecond. In a preferred embodiment, a wavelength tuning mirror is dithered at dither rates of more than 500 dithers per second in phase with the repetition rate of the laser. In one case, the piezoelectric drive was driven with a square wave signal and in a second case it was driven with a sine wave signal. In another embodiment, the maximum displacement was matched on a one-to-one basis with the laser pulses in order to produce a desired average spectrum with two peaks for a series of laser pulses. Other preferred embodiments utilize three separate wavelength tuning positions producing a spectrum with three separate peaks.